Viral upper respiratory infection (URTI): This is the most common cause of a prolonged cough in toddlers, as they are frequently exposed to various viruses that can cause URTIs, and their immune systems are still developing.

Asthma: A common condition in children that can cause chronic cough, especially if triggered by viral infections or allergens.
Allergic rhinitis: Allergies can lead to chronic cough due to postnasal drip.
Reactive airway disease: Similar to asthma, this condition involves airway hyperresponsiveness and can cause cough.
Pertussis (whooping cough): Although less common due to vaccination, it's still a possibility, especially if the child is not up to date on vaccinations or has been exposed to someone with pertussis.

Pneumonia: Bacterial pneumonia can present with a prolonged cough and is a serious condition that requires prompt antibiotic treatment.
Tuberculosis (TB): Although rare in many areas, TB is a critical diagnosis not to miss due to its serious consequences and the need for specific treatment.
Foreign body aspiration: This is an emergency that can cause a sudden onset of cough and respiratory distress.
Congenital heart disease: Certain heart defects can lead to chronic cough due to pulmonary congestion.

Cystic fibrosis: A genetic disorder that leads to chronic respiratory infections and cough.
Primary ciliary dyskinesia: A rare condition affecting the cilia in the respiratory tract, leading to chronic respiratory infections and cough.
Immunodeficiency: Conditions that impair the immune system can lead to recurrent or chronic infections, including those causing cough.
Gastroesophageal reflux disease (GERD): Can cause a chronic cough in toddlers due to acid reflux irritating the airways.